Re: [PATCH] net/virtio: fix Rx checksum calculation

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 18 Dec 2024 10:20:47 +0100
Olivier Matz <olivier.matz@6wind.com> wrote:

> Hi,
> 
> On Wed, Dec 18, 2024 at 09:59:05AM +0100, Maxime Coquelin wrote:
> > Hi,
> > 
> > On 12/18/24 08:34, Wangyunjian(wangyunjian,TongTu) wrote:  
> > > > -----Original Message-----
> > > > From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
> > > > Sent: Tuesday, December 17, 2024 11:33 PM
> > > > To: dev@dpdk.org
> > > > Cc: Olivier Matz <olivier.matz@6wind.com>; Maxime Gouin
> > > > <maxime.gouin@6wind.com>; Maxime Coquelin
> > > > <maxime.coquelin@redhat.com>
> > > > Subject: [PATCH] net/virtio: fix Rx checksum calculation
> > > > 
> > > > From: Olivier Matz <olivier.matz@6wind.com>
> > > > 
> > > > If hdr->csum_start is larger than packet length, the len argument passed
> > > > to rte_raw_cksum_mbuf() overflows and causes a segmentation fault.
> > > > 
> > > > Ignore checksum computation in this case.
> > > > 
> > > > CVE-2024-11614
> > > > 
> > > > Fixes: ca7036b4af3a ("vhost: fix offload flags in Rx path")
> > > > 
> > > > Signed-off-by: Maxime Gouin <maxime.gouin@6wind.com>
> > > > Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> > > > Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
> > > > ---
> > > >   lib/vhost/virtio_net.c | 3 +++
> > > >   1 file changed, 3 insertions(+)
> > > > 
> > > > diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c
> > > > index d764d4bc6a..69901ab3b5 100644
> > > > --- a/lib/vhost/virtio_net.c
> > > > +++ b/lib/vhost/virtio_net.c
> > > > @@ -2823,6 +2823,9 @@ vhost_dequeue_offload(struct virtio_net *dev,
> > > > struct virtio_net_hdr *hdr,
> > > >   			 */
> > > >   			uint16_t csum = 0, off;
> > > > 
> > > > +			if (hdr->csum_start >= rte_pktmbuf_pkt_len(m))
> > > > +				return;
> > > > +  
> > > 
> > > The hdr->csum_start does two successive reads from user space to read
> > > a variable length data structure. The result overflow if the data structure
> > > changes between the two reads.
> > > 
> > > We can prevent double fetch issue by using the temporary variable csum_start.  
> 
> This is an interesting remark, thanks!
> 
> However, in practical, I'd say that the hdr->csum_start is fetched in a register
> only once if using optimized compilation, because the compiler has no reason to
> think that hdr->csum_start can be modified.
> 
> Olivier

True, but security never depend on optimization.
Needs a fetch and compiler barrier to be truly safe against compilers.